General Description
(-)-2,3-
O-Isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane is a
C2 chelating diphosphine ligand for transition metal complexes during asymmetric catalysis.
Synthesis
In a 250 mL three-necked round-bottomed flask fitted with a PTFE stirrer, a coated stirring bar, a thermometer and its adapter, a reflux condenser with a gas inlet at the top, and a device with a 60 mL dosing funnel and PTFE Claisen fittings were assembled. The sample port of the plug valve was heated using a hot air gun and simultaneously purged with a rapid nitrogen stream to degas. 16.0 g (36.1 mmol) of (-)-1,4-di-O-toluenesulfonyl-2,3-O-isopropylidene-L-threitol (CAS No. 37002-45-2, also known as bis(toluenesulfonate)) was added to the reactor and purged with a nitrogen purge for an additional 30 min. Bis(toluene sulfonate) was dissolved in 100 mL of degassed anhydrous tetrahydrofuran. The sodium diarylphosphide solution prepared in step 2) was transferred to a 60 mL addition funnel through a cannula and added dropwise at a rate to a rapidly stirred solution of bis(toluene sulfonate), keeping the reactor temperature below 40 °C. Upon addition of the electrophilic reagent, the solution was reddish-orange in color and the diaryl phosphide salt disappeared rapidly to form a beige suspension. After dropwise addition, the oil bath was placed below the reactor and heated to 50 °C for 45 min to complete the phosphide coupling step. Subsequently, the Claisen adapter assembly was replaced with a short-range distillation head and approximately 50% of the tetrahydrofuran was removed by distillation. Phase separation was achieved by adding 100 mL of degassed heptane and water. The two-phase solution was filtered through an inert glass sintered funnel to remove carbon black and the lower aqueous phase was removed through a cannula. The upper organic phase was washed with 50 mL of degassed water and the aqueous phase was again removed through a cannula. Next, the organic heptane phase was extracted with 100 mL of anhydrous degassed acetonitrile, the lower acetonitrile phase was removed through a cannula, and the extraction was repeated once more with 50 mL of degassed acetonitrile. The acetonitrile phases were combined and transferred to a single-necked round-bottomed flask with a polytetrafluoroethylene stopcock valve and 24/40 standard conical ground glass fittings. The volatiles were removed on a rotary evaporator to give a viscous orange-brown glassy solid, which was further dried under vacuum. In the drying oven, the plug valve was replaced with a PTFE Claisen adapter with reflux condenser and gas inlet. The glassy solid was dissolved in 100 mL of degassed methanol, introduced via cannula or syringe, and heated to reflux to completely dissolve the ligand. Heating was stopped and the flask was immersed in an ice bath and cooled to -10 °C to give a white to off-white/beige solid. The solid DIOP ligand was collected through a glass sintered Schlenck filter, washed with cooled methanol, and dried overnight under vacuum. A final 11.7 g (23.5 mmol, molecular weight 498.34 g/mol) of (-)-DIOP ligand was obtained. The ligand was analyzed by 1H NMR (CDCl3: δ = 7.58-7.29 ppm, multiple peaks, 20H, aromatic; δ = 4.0-3.8 ppm, double peaks, 2H, -CH-; δ = 2.50-2.25 ppm, multiple peaks, 4H, -CH2-; δ = 1.35 ppm, single peaks, 6H, -CH3), 13C NMR (data not shown but consistent with the target product) and 31P NMR (CDCl3: δ=-22.87 ppm, single peak) for characterization. The molar percent P purity was >99%, and the total phosphorus content was determined by ICP to be 12.2 wt% P (theoretical value of 12.4 wt% P), indicating a chemical purity of approximately 98.4%, with a molecular formula of C31H32P2O2 and a molecular weight of 498.34 g/mol.
Purification Methods
It has been recrystallised from *C6H6/pet ether. After 2 recrystallisations from EtOH, it was pure by TLC on silica gel using Me2CO/hexane as solvent. [Kagan & Dang J Am Chem Soc 94 6429 1972.] Lanthanide shift reagents See in “Aliphatic Compounds”, Chapter 4, europium (III) acetate above and Eu(tmc)3 and Eu(tfc)3 below.
